Capacitive fill level measuring devices are known in the state of the art. The measuring principle is based on the fact that a probe unit, which is either a rod or a cable, and a second probe or the wall of the container in which the medium is located form the two electrodes of a capacitor, with the medium as a dielectric. Since the capacitance of this capacitor is, among other things, dependent on the fill level of the medium, the fill level can be deduced from the capacitance. Different options for measuring the capacitance are set forth, for example, in the Offenlegungsschrifts DE 101 57 762 A1 or DE 101 61 069 A1 of Endress+Hauser. For measuring, the probe is usually supplied with an exciter signal in the form of an electrical alternating voltage of a predeterminable frequency. The received signal extractable from the probe unit as the response signal is, in turn, usually an electrical current signal, which, for evaluation, most often is converted into an alternating voltage. From the received signal, there is obtained the capacitance of the capacitor, and therefrom, the fill level. The probe unit is most often coated with an electrically insulating layer, which permits continuous measuring of the fill level in the conductive media. An embodiment of the insulation is described, for example, in the Offenlegungsschrift DE 10 2005 053 330 A1.
A known problem lies in the fact that the medium can cling to the probe unit and an accretion forms. Such an accretion usually corrupts the measurement results or even prevents measurement. Further problematic is that the measured capacitance value not only depends on fill level, but also on the dielectric constant and the conductivity of the medium. Since the conductivity is influenced e.g. by temperature or humidity, these dependencies lead to measurement uncertainties, or limitations regarding application. Furthermore, the geometry of the container and, for example, accretion on the probe unit, also affect the measured values.
Another problem lies, due to the multiple dependence of the variables involved, in the associating of the capacitance value determined by the measuring and the fill level value actually of interest. Therefore, a calibration is most often required, in the case of which, after installation of the measuring device, different fill levels are brought about with the medium to be measured, and the capacitance values resulting in such case are saved. Such a calibration is, however, very complicated, and stands in the way of direct start-up of the measuring device following installation. Regarding this topic, the patent DE 195 36 199 C1 and the publication WO 2006/034959 A2 can be mentioned.
Furthermore, a problem lies in the area of the combining of probe unit and the electronics unit. The probe unit can, depending on the type of application and the need to be fulfilled, have markedly different lengths, this thus also meaning that, in each case, a markedly different bandwidth accompanies the capacitance values that can occur. The electronics unit nonetheless is designed for the maximum occurring value for the probe capacitance. This thus references the longest probe, and also the tolerances which can occur, for example, in the case of the insulation layer. Probe lengths lie most often between a few centimeters and about 30 meters. Equally, the tolerances of the insulations can also be very large. This leads to a large measuring range being provided in the electronics unit, which, however, is not fully utilized in all combinations of the electronics unit with different probe units. The actual measurement is most often implemented only with a markedly smaller measuring range. The resolution and the sensitivities in principle possible therewith, are, thus, not fully utilized. The susceptibility to EMC disturbances likewise increases therewith. A possibility is subdivision into a number of measuring ranges.